raqim-kashida 0.1.1

//! Grapheme clustering and Arabic joining analysis.
//!
//! Segments a word into UAX #29 grapheme clusters, tags each with its base
//! codepoint's Joining_Type / Joining_Group, and derives the joining
//! behavior: joined runs, joins-left/right, and positional form.

use icu_properties::props::{JoiningGroup, JoiningType};
use icu_properties::CodePointMapData;
use icu_segmenter::GraphemeClusterSegmenter;

pub(crate) const KASHIDA: char = '\u{0640}';

#[derive(Clone, Copy, Debug)]
pub(crate) struct Grapheme {
    pub(crate) base_codepoint: u32,
    pub(crate) joining_group: JoiningGroup,
    pub(crate) joining_type: JoiningType,
}

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub(crate) enum JoiningForm {
    Isolated,
    Initial,
    Medial,
    Final,
}

// A bare tatweel is a one not carrying a small alef or hamza above/below.
// These can potentially be stripped.
pub(crate) fn is_bare_tatweel_at(chars: &[char], k: usize) -> bool {
    if chars[k] != KASHIDA {
        return false;
    }
    let joining_types = CodePointMapData::<JoiningType>::new();
    for &c in &chars[k + 1..] {
        if joining_types.get(c) != JoiningType::Transparent {
            break;
        }
        if matches!(c, '\u{0670}' | '\u{0654}' | '\u{0655}') {
            return false;
        }
    }
    true
}

pub(crate) fn split_graphemes(word: &str) -> Vec<Grapheme> {
    let joining_types = CodePointMapData::<JoiningType>::new();
    let joining_groups = CodePointMapData::<JoiningGroup>::new();
    let boundaries: Vec<usize> = GraphemeClusterSegmenter::new().segment_str(word).collect();
    let mut out = Vec::with_capacity(boundaries.len().saturating_sub(1));
    for pair in boundaries.windows(2) {
        let cluster = &word[pair[0]..pair[1]];
        let base = cluster.chars().next().expect("non-empty cluster");
        let mut joining_type = joining_types.get(base);
        // A ZWJ/ZWNJ change joining type.
        if cluster
            .chars()
            .skip(1)
            .any(|c| joining_types.get(c) == JoiningType::NonJoining)
        {
            joining_type = match joining_type {
                JoiningType::DualJoining | JoiningType::JoinCausing => JoiningType::RightJoining,
                JoiningType::LeftJoining | JoiningType::Transparent => JoiningType::NonJoining,
                other => other,
            };
        } else if joining_type == JoiningType::DualJoining
            && cluster
                .chars()
                .skip(1)
                .any(|c| joining_types.get(c) == JoiningType::JoinCausing)
        {
            joining_type = JoiningType::JoinCausing;
        }
        out.push(Grapheme {
            base_codepoint: base as u32,
            joining_group: joining_groups.get(base),
            joining_type,
        });
    }
    out
}

pub(crate) fn is_joining_type(joining_type: JoiningType) -> bool {
    matches!(
        joining_type,
        JoiningType::DualJoining
            | JoiningType::RightJoining
            | JoiningType::LeftJoining
            | JoiningType::JoinCausing
    )
}

pub(crate) fn joins_left(graphemes: &[Grapheme], index: usize) -> bool {
    let cur = &graphemes[index];
    if !is_joining_type(cur.joining_type) || cur.joining_type == JoiningType::RightJoining {
        return false;
    }
    match graphemes.get(index + 1) {
        Some(next) => {
            is_joining_type(next.joining_type) && next.joining_type != JoiningType::LeftJoining
        }
        None => false,
    }
}

pub(crate) fn joins_right(graphemes: &[Grapheme], index: usize) -> bool {
    let cur = &graphemes[index];
    if !is_joining_type(cur.joining_type) || cur.joining_type == JoiningType::LeftJoining {
        return false;
    }
    let prev = index.checked_sub(1).map(|i| &graphemes[i]);
    match prev {
        Some(p) if is_joining_type(p.joining_type) => p.joining_type != JoiningType::RightJoining,
        _ => false,
    }
}

pub(crate) fn form_of(graphemes: &[Grapheme], index: usize) -> JoiningForm {
    let right = joins_right(graphemes, index);
    let left = joins_left(graphemes, index);
    match (right, left) {
        (true, true) => JoiningForm::Medial,
        (false, true) => JoiningForm::Initial,
        (true, false) => JoiningForm::Final,
        (false, false) => JoiningForm::Isolated,
    }
}

// Joined runs: maximal spans of joining letters connected together.
pub(crate) fn joined_runs(graphemes: &[Grapheme]) -> Vec<Vec<usize>> {
    let mut out: Vec<Vec<usize>> = Vec::new();
    let mut current: Vec<usize> = Vec::new();
    for (i, g) in graphemes.iter().enumerate() {
        if g.joining_type == JoiningType::Transparent {
            continue;
        }
        if !is_joining_type(g.joining_type) {
            if !current.is_empty() {
                out.push(std::mem::take(&mut current));
            }
            continue;
        }
        if let Some(&last) = current.last() {
            let prev = &graphemes[last];
            let prev_joins_forward = prev.joining_type != JoiningType::RightJoining;
            let cur_joins_backward = g.joining_type != JoiningType::LeftJoining;
            if !prev_joins_forward || !cur_joins_backward {
                out.push(std::mem::take(&mut current));
            }
        }
        current.push(i);
    }
    if !current.is_empty() {
        out.push(current);
    }
    out
}